Cells of mammalian heart muscle are enveloped and connected by an extensive and varied skeletal network of inter- and peri-cellular bundles of collagen that exist in addition to intercalated discs. Integrated into this collagen wave are elastic fibers that are often helically wound around the myocytes. Superimposed on this network is a fine lattice comprised of smaller components, including individual collagen fibrils, microfibrils (10-17 nm in diameter), microthreads (3-7 nm in diameter), polyanionic granules, and polyanionic "amorphous" material. The objective of this study is to further characterize the structural and functional properties of these structures. In this study, we will define linkages in terms of structure of individual components and their interations (eg, stoichiometry and density of attachments) using standard and high voltage electron microscopy in conjunction with specialized fixation techniques. A model is proposed as a working hypothesis to direct our measurements and focus our interpretations of findings. We will develop a more complete model of these linkages and compare it with those developed for the extracellular matrix of other tissues in terms of "dynamic" and "static" systems. We will quantitate dynamic changes in angles of linkages relative to the cylindrical muscle cell's long axis during selected phases of the contractile cycle and in states of induced stretch using light microscopy and E.M. and relate configurations to sarcomere length. Identification of biochemical composition of components of the linkage system will be performed in situ using specific antibodies, fluorescently labelled for light microscopy and labelled with electron-dense markers for E.M.; special polycationic stains; and selective enzymatic digestion. Components will be categorized as "elastic" or "non-elastic" in conjunction with their geometrical localization relative to the contracting, moving cells. We propose that the network of linages between cells is distensible within limits but they and pericellular collagen "cuffs" limit over-stretching or over-shortening of heart muscle. Elastic components store energy generated during contraction (as stretched springs) and act in concert with elongating myocytes to produce rearrangemnt of cells during diastole of the heart.